Add support for parsing Taffy style types from string

[nicoburns]

Objective

• Implements the parsing side of Parse Style from (and serialize to?) CSS syntax #440
• Is used in Represent generated tests as XML rather than Rust code #926

Context

• Split out from Represent generated tests as XML rather than Rust code #926
• Continues/reimplements the abandoned [WIP] Parse Style from and serialize to CSS syntax #460

Notes

• This bumps MSRV to 1.71 because cssparser with default features depends on syn which requires 1.71. We could technically avoid this as we're not using default features. But it doesn't seem worth the effort given that 1.71 is from July 2023?
• This adds an (optional) dependency on cssparser which is MPL
• cssparser no longer has a hard dependency on proc-macro infra

Todo

• Blocked on Release v0.37.0 servo/rust-cssparser#428
• Consider bringing back the from_keywords! macro from [WIP] Parse Style from and serialize to CSS syntax #460
• Figure out error handling. Currently diagnostics on failure are not good.
• Work out whether the FromCss trait (and thus the cssparser dependency) should be public (for now it's private, we can make it public later if desired).

Future work

• Add a FromStr implementation for taffy::Style (that supports parsing a semicolon-delimited CSS style block)

[nicoburns]

@SimonSapin I already merged this, but this is effectively a revival of your earlier PR (#460). And your review (especially around correct cssparser usage) would be welcomed.

[SimonSapin]

This is nice to see! I can answer specific questions if you have them but sorry, it looks like a full review of a PR this size would take more time than I have right now.

[nicoburns]

No worries! Just wanted to give you the chance to input if you wanted to :)